Natural repeating variations in the climate.
An oscillation is a repetitive variation around a "normal" or between two or more states. The rise and fall in sea level because of tidal forces, and the tidal current that is created, is an oscillating movement.
A climate oscillation is not the same as a climate change. Climate change most often refers to changes, that will not automatically correct themselves. Oscillations are variations that happen regularly, and are not permanent.
Understanding and identifying natural variations in the Earth's climate is important for determining whether observed climate changes are unusual and perhaps caused by humans, or if they are within the natural system of variation.
It can be difficult to distinguish climate changes from natural climate oscillations. Especially if the time span of a climate oscillation is very long.
Natural climate oscillations include El Niño, a ocean-atmosphere phenomenon that occurs regularly with an interval between 2 to 7 years. During El Niño the temperature of the central and eastern pacific ocean rises above normal. The phenomenon have wide ranging consequences for the climate for much of the planet.
Natural climate oscillations may temporarily keep temperatures in check, and even result in a cooling of areas that were expecting the effects of global warming. That does not necessarily mean the estimated long term temperature rise predicted by scientists is false, but rather that the there will be an interaction between global warming and natural climate variations.